Colorimetric assay based on iron(iii) ions triggering the aggregation of a poly(tannic acid) coated Au nanocomposite for carbonic anhydrase II detection

Abstract

Herein, several components including mesoporous silica nanoparticles (MSNs) as a reservoir to load iron(III) ions for triggering a color change and a gold nanoparticle (AuNP) as an imaging agent prepared and stabilized using poly(tannic acid) (PTA) were combined together to successfully construct a kind of colorimetric assay for detection of protein. Iron(III) ion-loaded MSNs with an amine-modified surface were capped with the probe-labelled PTA-based shell@core AuNP (AuNP@PTA) through electrostatic interaction, forming probe-labelled PEGylated AuNP@PTA@Fe³⁺-loaded MSNs. The stronger interaction of the target analyte and the probe molecule would pull the gatekeeper AuNP@PTA away from the surface of the MSNs. The released iron(III) ion would coordinate with PTA, leading to AuNP aggregation for a visual color change. In this study, isozyme carbonic anhydrase II (CA II) and 4-sulfonamidebenzoic acid (SABA) were selected as the protein analyte and probe molecule. The SABA-labelled PEGylated AuNP@PTA@Fe³⁺-loaded MSNs showed good recognition specificity and sensitivity towards CA II with a visual detection limit of 50 nM, and the limit of detection (LOD) for the AuNP@PTA@Fe³⁺-loaded MSNs was about 15 nM using a signal-to-noise ratio of 3 with a UV-vis technique. Such proposed nanocomposites exhibited great potential as sensors in medical diagnostics and biological systems without the need for any separation process.